This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Transit time ultrasonic flowmeters function by measuring the time it takes for ultrasonic signals to propagate between transducers placed at different locations around or inside a meter body through which the fluid is conveyed. The cross-sectional area of the flow passage through the meter body, the path lengths separating pairs of transducers, and the angle to the flow axis formed between pairs of transducers all play a part in the relationship between the measured times and the rate of flow.
The relationship between the measured transit times and the meter geometry is normally derived from the principle of operation. Calculation of the flow rate then depends on numerical inputs that characterize the geometry of the meter body. These inputs may be derived from design information or based on physical measurement of the meter body, and in the case of high-accuracy ultrasonic meters will normally include calibration parameters that act to correct errors arising from assumptions and/or geometric uncertainties.
In applications where the process fluid may contain contaminants or where the meter body may be prone to erosion or corrosion, the effective geometry of the meter body can be altered. If a change in geometry occurs and is not corrected by calibration or some other means, then this may result in a flow measurement error. For example, if the cross-sectional area of the flow passage is reduced, then the flow velocity may increase. And if the computation of flow rate does not account for the reduced area, the flow rate may be over-registered.
It is known in the field of ultrasonic flow metering that alteration of the internal condition of an ultrasonic meter may affect the accuracy of the meter in the manner described above. See, for example, John Lansing, Dirty vs. Clean Ultrasonic Gas Flow Meter Performance, AGA Operations Conference, Chicago, Ill. 2002. Work has also been carried out with the aim of proving self-diagnosing capability for ultrasonic meters in such situations see for example, John Lansing, How Today's USM diagnostics solve metering problems, North Sea Flow Measurement Workshop, 2005, Tonsberg, Norway.
In oil pipelines various potential contaminants exist, for example, in the form of paraffin wax, asphaltines and inorganic scale. In gas pipelines black powder contamination is well known. The nature of black powder contamination is varied and uncertain but may be from mill scale or corrosion products mechanically mixed or chemically combined with any number of contaminants such as water, liquid hydrocarbons, salts, chlorides, sand, or dirt. Chemical analyses of black powder contamination have revealed that it typically consists mainly of a mixture of iron oxides and iron sulphides. Furthermore, pipelines may also contain water, including salt water, which can lead to corrosion of the internal parts of ultrasonic flowmeter bodies.
Therefore, it is desirable that the internal surfaces of ultrasonic meters be resistant to corrosion and the deposition of contaminants.